This section provides background information related to the present disclosure which is not necessarily prior art.
Demand for transportation fuels is predicted to grow at a faster rate than energy use in any other sector over the next 25 years as the economies of developing nations grow. Diversifying the pool of transportation fuels is necessary to offset the many problems associated with increased demand. Alternative fuels, such as biofuels, can reduce greenhouse gas emissions and, if domestically sourced, can provide increased energy security. New engine technologies and combustion regimes being developed to reduce emissions and increase engine efficiency are changing the combustion process within engines. These new engines can greatly benefit from fuels that are optimized for use in them.
The development and production of new fuels is typically done in a laboratory setting, making it difficult and costly to produce large quantities of fuel. One of the most important fuel characteristics for diesel engines is its ignition quality, which gives some indication of how readily the fuel autoignites. Current ignition characterization tests require large amounts of fuel and very specialized and expensive equipment. The ignition quality of diesel fuel is characterized by the cetane number, which is based on testing the fuel in a Cooperative Fuels Research (CFR) engine (see, e.g., https://www.asme.org/getmedia/ffedc33f-7e2b-4775-95ec-2f633ddc16f6/50-Cooperative-Fuel-Research-Engine-1928.aspx).
The CFR engine is an engine that was originally developed in 1929 specifically for fuels testing. Operation of the CFR engine with the test fuel is compared to operation with a mixture of two standard fuels (cetane and isocetane, also known as n-hexadecane and 2,2,4,4,6,8,8-Heptamethylnonane, as defined in ASTM D-613). The percentage of cetane in the mixture that gives comparable operation defines the cetane number of the test fuel.
Recently, the ignition quality tester (IQT) was developed to decrease the testing effort and the volume of fuel required for determining the ignition quality of a given test fuel. In this test, fuel is injected into a constant volume chamber filled with air at high pressure and temperature; the time between injection of the fuel and the occurrence of ignition is then used to determine the derived cetane number of the test fuel as defined in ASTM D-6890. This method requires on the order of 100 ml of fuel, which can still be a prohibitively large quantity for fuels generated in a research laboratory.
Another issue with CFR and IQT ignition quality tests is that they make use of legacy injection technology. The ignition delay determined in an engine or the IQT is a function of the time for the fuel to evaporate and mix, as well as a chemical time-delay. The older injection technology found in these legacy tests uses lower injection pressures, producing larger fuel droplets than today's high pressure common rail injection systems. Therefore, the vaporization time delay in these tests is inconsistent with that found in engines using modern injection technology.
The cetane number enables comparison of diesel fuel from all over the globe, and its allowable range can vary greatly from country to country. This variation is an issue for vehicles that are calibrated with a particular standard cetane number in one country and then moved to a country with a different standard (if that particular country even has a standard at all). This situation occurs often in the military, which uses vehicles all over the world and must deal with large variations in fuel quality. Operating a diesel engine with fuel that differs in cetane number from the engine's baseline calibration fuel can lead to decreased engine efficiency, increased harmful emissions, failure to achieve fuel ignition, or even engine damage.
On-board fuel characterization would improve operation of engines when fuel properties vary from the baseline fuel that the engine is calibrated for. The engine control unit (ECU) could be updated with information about fuel ignition quality that is currently in the vehicle's tank and make appropriate corrections to the injection timing, exhaust gas residuals, or other engine parameters, resulting in more efficient operation and cleaner (i.e., reduced emissions) operation. An engine equipped in this manner could then readily deal with a range of fuels, including biofuels, despite being calibrated with a different fuel. This flexibility has especially significant implications for military vehicles that see wide-ranging fuel variation from country-to-country, as well as the normal variations that occur season-to-season in the United States, and from region-to-region within the United States at a given time of year.
Similar to the cetane number used to characterize diesel fuel, the octane number is used to characterize fuel for spark-ignited engines. The CFR engine is also used to determine a fuel's octane number in ASTM D-2699 and ASTM D-2700. The issues that make testing new diesel fuels difficult and costly also plague fuels for spark-ignited engines. There is a similar need to develop new methods of testing properties of fuels for spark-ignited engines that require low volumes of fuel and can be carried out in real time on-board the vehicle.